On the other hand, if you do need a new machine right now, you should get one now. Desktop updates aren't expected until early-mid 2015, as far as most rumors are concerned. The Mac Mini uses mobile components that the rMBP uses, so it may be updated earlier, but I can almost guarantee it won't be this September. Depending on your needs, the best options for your money are below. This is personal opinion, so take it with a grain of salt. I am assuming you don't need portability, so the options I chose don't attribute any value to that feature.
Low-end options ($500-$700):
Baseline, used Mac Mini
This is a great budget computer, and it is still fairly competitive considering its age. You can find them used on ebay for less than $500. You can also pick up a nice 256GB SSD for around $110, and put the 500 GB HDD into a USB 3.0 external enclosure. I would recommend this computer if you are tight on budget and already have a screen/keyboard/mouse.
Low/Mid-end options ($750-$900):
Baseline 2014 MacBook Air
Quad-Core, Baseline, Refurbished 2012 Mac Mini
The MBA is really the only computer in this range that I would recommend for a consumer. You can find them new at BestBuy for as little as $750 with student discount. Any comparable mini will be slower for most tasks. If you really need a quad-core and you are tight on money, get the mini.
http://store.apple.com/us/product/G0NP1LL/A/refurbished-mac-mini-26ghz-quad-core-Intel-Core-i7
It has HD 4000 graphics though, so if you are doing any 3D graphics work I would really consider an iMac, and even a MBA will be more capable on that end. The MBA also has PCIE flash, which is a godsend for the UI. If you don't do professional work, the MBA is really your best option at this price range. This is not a great price range for professionals just because of the lack of graphics and flash storage on the Mini.
Mid-end options ($1050-$1400):
Refurbished 2012/2013 21.5" iMac
Don't worry about 2012 vs 2013 models, as they are almost identical internally, and completely identical externally. There is a 2012 model with a 1TB Fusion Drive for $1229 right now. It is the one I would recommend if you are tight on money. It will perform well for professional work and consumer tasks as well. This is a great mid-range computer.
http://store.apple.com/us/product/G0MP0LL/A/refurbished-imac-27ghz-quad-core-Intel-Core-i5
The next one up is a 2013 model with better graphics, processor, and PCIE flash with the Fusion Drive. It is $210 more, so if you need the power it will be a worthwhile investment. You won't notice any difference for most tasks, but the value of the component upgrades is easily $210 if you need it.
http://store.apple.com/us/product/G0PE1LL/A/refurbished-215-inch-imac-29ghz-quad-core-Intel-Core-i5
Mid/High-end options ($1500-$1800):
N/A
There isn't really anything in this price range worth purchasing. It is an awkward abyss in Apple's lineup where the computers are too expensive for the power they provide.
High-end options ($1850-$3000):
Refurbished 27" iMac
The sky is the limit in this category. The 27" iMac is a significantly more powerful computer than the 21.5" iMac, and the display is jaw dropping. On the low end of this range is the $1870 refurbished 2013 27" iMac. It is actually a really solid computer, and it is one of the reasons why the $1500-$1800 price range is irrelevant. The 775M has more than triple the graphics power of the Iris Pro, it has upgradeable RAM, and as mentioned before, the display is gorgeous. This is my highest recommended 27" model.
http://store.apple.com/us/product/G0PG1LL/A/refurbished-27-inch-imac-34ghz-quad-core-Intel-Core-i5
If you aren't into gaming or graphics work, this model comes with 512GB of PCIE flash for $1950. It is insanely fast for any consumer task, and the 755M is great, it just isn't that great if you are a power user.
http://store.apple.com/us/product/G0PF2LL/A/refurbished-27-inch-imac-32ghz-quad-core-Intel-Core-i5
This one comes with the 780M and an i7 processor for $2300. Really only get this one if you can name a professional program you use 20+ hours a week.
http://store.apple.com/us/product/G0PG4LL/A/refurbished-27-inch-imac-35ghz-quad-core-Intel-Core-i7
This is the 2012 maxed out model. This is exactly the same externally as the 2013 model. If you aren't a professional user, but the model above is still tempting you, get this one instead for $1950.
http://store.apple.com/us/product/G0MS5LL/A/refurbished-27-inch-imac-34ghz-quad-core-Intel-Core-i7
Final Note:
There are higher end options, but I wouldn't consider getting them unless you have $2500+ just lying around. You should probably look into getting a nMP if you hit that price range, or waiting around for a 2015 model to come out. All the computers above are completely viable, and will be competitively priced with new computers coming out in 2015. If you need a computer now, you can't go wrong with any of the recommended computers above.
Best,
Matt